In a battery, a separator is used to separate a positive electrode and a negative electrode, to prevent a short circuit therebetween, and further, to hold an electrolyte thereon and enable a smooth electromotive reaction.
Recently, the space allotted for the battery in electronic equipment has become smaller, due to the need for miniaturization and weight-saving. Nevertheless, the performance requirement for the smaller battery is the same as or higher than that for a conventional battery, and therefore, it is necessary to enhance the capacity of the battery, and to increase the amounts of active materials in the electrodes. Accordingly, the volume allotted in the battery for the separator must be reduced.
As such a separator having a reduced volume allotted in the battery, the present applicant discloses “a battery separator consisting essentially of a nonwoven fabric having a substantially unilayered structure, wherein an apparent total surface area of fibers per surface density of the nonwoven fabric is 20 m2 or more, a thickness of the nonwoven fabric is 0.1 mm or less, a uniformity index of the nonwoven fabric is 0.15 or less, and the nonwoven fabric contains fine fibers having a fiber diameter of 4 μm or less” (patent reference 1).
Further, “a battery separator characterized in that a layer made of a wet-laid papermaking web and containing heat-adhesive fibers (B) at a mass ratio of 0.1 or more with respect to the whole mass of fibers constituting the wet-laid papermaking web is laminated on at least a surface of a wet-laid nonwoven fabric in which heat-adhesive fibers (A) are contained at a mass ratio of 0.1 or more with respect to the whole mass of constituent fibers, constituent fibers are heat-adhered with the heat-adhesive fibers (A), the interlayer between the wet-laid nonwoven fabric and the wet-laid papermaking web layer is integrated by heat-adhesion with at least one kind of fibers selected from the heat-adhesive fibers (A) and the heat-adhesive fibers (B), and a peeling strength determined by JIS-L-1086, 6.19.1 is 0.1 to 5 N” is disclosed in patent reference 2.
Furthermore, the present applicant discloses “a nonwoven fabric comprising an entangled fiber web containing a low-density region, wherein a fibrous material is adhered to at least the low-density region, and an average pore size is 12 μm or less” (patent reference 3).    [patent reference 1] Japanese Unexamined Patent Publication (Kokai) No. 2002-124239 (claim 1, Examples, and the like)    [patent reference 2] Japanese Unexamined Patent Publication (Kokai) No. 2002-124242 (claim 1 and the like)    [patent reference 3] Japanese Unexamined Patent Publication (Kokai) No. 7-272709 (claim 1, paragraph [0037], and the like)